The stenciling process used to etch stone faces in the monument industry involves sandblasting stone areas outlined by a rubber stencil sheet. Stencil is basically a thick piece of rubber (approximately 15-45 millimeters) coated with a pressure sensitive adhesive. After the rubber stencil is adhesively affixed to the stone, lettering and decorative ornamentation are drawn or transferred onto the rubber stencil. The appropriate portions are then cut and removed leaving a design template. After sandblasting, the design template is peeled away from the stone face and the etched design remains.
However, the surface of stone monuments, where the stone is most commonly granite, is frequently rough and pitted either as a result of the quarrying cutting process or as a consequence of a prior sandblasting step. In order to create a smooth surface on which the rubber stencil will adhere, the monument surface is primed with a coating known as stencil (a.k.a. sandblast) filler. Typically stencil filler is a slightly tackified rubber solvated in a common organic solvent, such as toluene or heptane. Solvent formulated stencil fillers having a solvating medium are brushed on the stone, and dry to form a slightly tacky rubber surface to which the stencil can facilely adhere. Following application of the stencil and the sandblasting process, the stencil is stripped off. Depending on various factors, such as the degree of surface roughness of the underlying stone, there will be varying amounts of residual stencil filler coating left on the stone after the stencil is removed. This residual stencil filler coating has to be cleaned off the stone. Solvent formulated stencil filler coatings have traditionally been removed using an organic solvent, such as unleaded gasoline (which contains toluene and heptane as well as multiple other components) or varsol. Generally speaking, much more solvent is required to clean the stone than was present as the solvating medium in stencil filler.
In the recent past, seepage of organic solvents into ground water has been identified as a significant environmental issue. In response, restrictive local, state, and federal regulations on ground water have been passed which serve to limit, and ultimately eliminate. the use of most common organic solvents. In some states, for instance California, in addition to the ground water regulations, there are stringent air emission standards which read on the use of volatile organic solvents. As previously alluded to, many of these volatile organic solvents are commonly utilized as the solvating medium in stencil fillers, and also as the solvent used to clean the stone.
In addition to organic solvents, acids, such as muriatic acid, have been used to remove the solvent formulated stencil filler coating from the surface of the stone monument. However, regulatory agencies monitor the pH level of waste water as well as organic solvent content.
In summary, a solvent formulated stencil filler is a flammable source of VOC air emissions during drying. Secondly, a conventional solvent formulated stencil filler dries to a coating which is substantially impervious to water, and requires the use of solvents to clean off following sandblasting.
There exists a need for a water based stencil filler which doesn't contain volatile organic solvents, where the water based stencil filler is stipulated interchangeable with the solvent formulated stencil filler. The dried primer coating derived from a water based stencil filler must have adhesion properties which are comparable to a dried primer coating derived from a solvent formulated stencil filler, as there are no changes anticipated for the pressure sensitive adhesive on the stencil sheet. A further consideration is that the dried primer coating derived from a water based stencil filler can be removed from the surface of a stone monument without the use of organic solvents or acids, and that no hazardous waste stream is produced as a consequence of the cleaning process. The limitation of the stipulated interchangeability constrains the water based stencil filler such that it must dry at room temperature in a relatively short time-frame without the use of auxiliary heat. Granite, the preferred stone for monuments has a very high heat capacity ("sweats"), and can contain several percent of water by weight. In light of the practical realities imposed by the properties of stone, one is lead to the conclusion that the drying process is such that it is highly unlikely that a water based stencil filler will ever completely dry. Therefore, a final consideration is that the water based stencil filler coating must attain stipulated interchangeability of the "dried" coating while water is still present on the order of several percent on a weight basis in the "dried" coating.
Schwartz 4,419,481 reads on a water based adhesive that when dried at 250 F for 2 minutes forms a pressure sensitive adhesive coating. Properties of a room temperature dried adhesive are not discussed, but can be anticipated to be significantly different. The Elberton Granite Association has a number of bulletins that teach the current technology of sandblasting, and more specifically the utilization of stencil filler. Evans et al 4,183,834 reads on a resinous polyelectrolyte, and Evans an al 4,477,613 reads on a carboxyl-containing rosin derived resinous composition having an acid value of from 30 to 150 and a softening point of from -25 C to 150 C, at least partially neutralized with a counter ion capable of carrying the resinous composition into solution or dispersion in water. In the instant invention, it was discovered that certain unanticipated properties of Evans et al 4,183,834 and 4,477,613, could be exploited and expanded upon to produce a substantially hydrophobic rubbery coating that, even as a dried filmic coating, would be rewettable in copious quantities of water.